\[ f(x) y''(x)+f(x) y(x)+y'(x)+y^{(3)}(x)=0 \] ✓ Mathematica : cpu = 0.0788075 (sec), leaf count = 84
\[\left \{\left \{y(x)\to c_3 e^{i x} \int _1^xe^{-2 i K[3]} \int _1^{K[3]}\exp \left (\int _1^{K[2]}(i-f(K[1]))dK[1]\right )dK[2]dK[3]+c_1 e^{i x}+\frac {1}{2} i c_2 e^{-i x}\right \}\right \}\] ✓ Maple : cpu = 0.329 (sec), leaf count = 36
\[ \left \{ y \left ( x \right ) ={{\rm e}^{ix}} \left ( \int \!{{\rm e}^{-2\,ix}} \left ( \int \!{\it \_C3}\,{{\rm e}^{\int \!i-f \left ( x \right ) \,{\rm d}x}}\,{\rm d}x+{\it \_C2} \right ) \,{\rm d}x+{\it \_C1} \right ) \right \} \]